The present invention relates to a mechanical transmission, and more particularly to the construction of a transmission unit of a transmission which includes dual clutches.
There are known mechanical automatic transmissions which use no torque converter, as transmissions for vehicles. In these mechanical automatic transmissions, operations (gear selection and shift) in a transmission of a manual transmission and engagement and disengagement of clutches are performed by an actuator, whereby automatic gear changes requiring no torque converters are enabled. The transmission includes a shift shaft which can be moved in a shifting direction and a selecting direction through sliding and rotating, a plurality of shift lugs (shift brackets) which are arranged in the selecting direction, and shift rails (rods) which connect the shift lugs and shift forks, respectively. Furthermore, a shift member (shifter), which is formed into a pawl shape, is provided on the shift shaft in such a manner as to project outwards, and a pair of claw portions is provided on each shift lug in such a manner as to be spaced apart from each other in the shifting direction. In addition, the shift shaft is moved by the actuator in the mechanical automatic transmission, whereby the claw portion of the shift lug is selectively pushed to be moved in the shifting direction by the shift member, so that the shift fork is operated to be moved selectively.
Furthermore, in the mechanical automatic transmissions, dual clutch type automatic transmissions having two clutches have been developed. In the dual clutch type automatic transmissions, from a state in which one gear is engaged via one of the clutches, a target gear is engaged via the other clutch, and when the rotational speed of the target gear comes to synchronize with that of the gear engaged via the one clutch, the one clutch is disengaged, whereby no neutral state takes place during the gearchange, and a gearchange is enabled in which there is virtually no interruption of power flow as the gearchange takes places.
In this way, in the dual clutch type automatic transmission, since shift operations for the two gears are performed sequentially during the gearchange, the movements of the shift member become complex, and as a result, there is caused a fear that the shifting time becomes long. To cope with this, there has been proposed a mechanical automatic transmission in which a space between a pair of claw portions provided on each shift lug is expanded in the shifting direction, so that a shift member can enter between the pair of claw portions on the shift lug for a target gear which is in a neutral position only by shifting the shift member from between the pair of claw portions of the shift lug which is in a shifted state in the selecting direction, the movements of the shift member being thereby simplified (refer to JP-A-2001-304411).
In JP-A-2001-304411, however, in order for the shift member to enter between the pair of claw portions when the shift member is moved to the selecting direction, the pair of claw portions must be disposed in such a manner that the space between the pair of claw portions is expanded relatively wide. Consequently, when a gear is engaged, since a distance over which the shift member is moved from its neutral position to the claw portion for abutment becomes relatively long, there is caused a fear that a quick shift is not attained.
In addition, in the event that the pair of claw portions are disposed in such a manner that the space therebetween is expanded relatively wide, force transmitted from the shift member to the claw portion becomes small, and the efficiency is deteriorated. Hereinafter, the reason for the deteriorated efficiency will be described using FIG. 9. FIG. 9 is a schematic diagram showing the construction of a shift mechanism of a shift lug according to the related art (JP-A-2001-304411).
As is shown in FIG. 9, in the mechanical automatic transmission described in JP-A-2001-304411, a construction is adopted in which a shift member 71 is caused to swing by rotation of a control shaft 70 so as to push a claw portion 73 of a shift lug 72 in a shifting direction. In the construction like this, when a space L1 between a pair of claw portions 73 is increased, the shift member 71 is caused to push the claw portion 73 while the shift member 71 is tilted largely towards the shifting direction. Assuming that a force exerted on the claw portion 73 by a rotational torque T of the control shaft 70 then is a tangential force F, a distance from a contact point a between the shift member 71 and the claw portion 73 to an axial center C of the control shaft 70 is a distance L2, and a tilt angle of a line which connects the contact point a with the axial center C towards the shifting direction is an angle α, a component of the tangential force F in the shifting direction, that is, a component force P1 which is a force which shifts the shift lug 72 in the shifting direction is obtained by the following equation (1).P1=F×COS α=T×COS α/L2  (1)
In the equation (1), in the event that the distance L2 is substantially constant irrespective of the angle α, it is found that when the angle α is increased within a range of 0 to 90 degrees, the component force PT is decreased. Consequently, in the event that the space L1 between the pair of claw portions 73 is increased as in JP-A-2001-304411, the force pushing the claw portion 73 towards the shifting direction is decreased, and hence, it becomes difficult to move the shift lug 72 in the shifting direction with good efficiency.